The Logitech Z-5500 5.1 speaker system comes with an external decoder and has exactly the inputs that you are looking for. The 3 pc inputs, 1 digital coax and 1 optical input. http://www.logitech.com/index.cfm/speakers_audio/home_pc_speakers/devices/224&cl=us,en
I actually have the exact same setup with my XBox and PC plugged into the same screen and I use the digital out on the XBox with the speakers.
You should also be able to pick them up for a really good price.